1. Field of the Invention
The present invention relates to an adaptor device for connecting a sheathed heater with a power supply terminal, and more particularly to an improvement of an adaptor device for connecting a heater wire of a sheathed heater such as a cable heater with a power supply lead wire, wherein the heater wire of the cable heater in the adaptor device is prevented from being broken.
2 . Description of the Prior Art
FIG. 1 shows a conventional adaptor device 2 for connecting an end of a heater wire 3 of a sheathed heater 1 of not greater than 7mm in outer diameter, for example, with an end of a lead wire 4 of a power supply, The end of the heater wire 3 is directly connected by welding with the end of the lead wire 4 in the adaptor device 2.
A reference numeral 6 denotes an insulating tube into which the ends of the heater wire 3 and the lead wire 4 are inserted, and a reference numeral 7 denotes a sleeve surrounding the insulating tube 6. A reference numeral 8 is an insulating powder material filled in the insulating tube 6 and the sleeve 7, 9 denotes a hermetically seal at each end of the sleeve 7, and 10 denotes a sheath of the sheathed heater 1. FIG. 2 shows a portion of the other conventional adaptor device 2, wherein the heater wire 3 is connected by a cylindrical intermediate terminal having a small inner diameter portion and a large inner diameter portion with the lead wire 4.
In such conventional adaptor devices, however, the heater wire 3 is liable to be broken in the adaptor device because the stress applied on the tip end of the heater wire 3 is increased due to the differences between the sleeve 7 of the adaptor device 2 and the lead wire 4 in the rate of thermal expansion. For example, if the sleeve 7 is larger in the rate of thermal expansion than the lead wire 4, an extendible force is applied on the tip end of the heater wire 3, whereas on the sleeve 7 is smaller in the rate of thermal expansion than the lead wire 4, a compressive force is applied on the tip end of the heater wire 3.